The present invention relates to an automatic wafer alignment system and more particularly to such a system which operates by locating pairs of oppositely-inclined diagonal features on the face of the wafer by analyzing a raster scanned video signal.
The present invention is adapted for accomplishing automatically a procedure which is heretofore been done manually. The procedure which has been automated is the alignment of a silicon wafer on an X-Y stage prior to the exposure of that wafer to produce a desired resist pattern. The resist pattern is used for subsequent processing as part of the semiconductor manufacturing procedure. While the system was developed particularly for use in conjunction with the projection exposure apparatus sold by the GCA Corporation of Bedford, Mass. under the trademark DSW Wafer Stepper, it should be understood that such alignment procedures are necessary with most resist exposing systems and that the present invention will be similarly useable with such systems.
In operation, the system of the present invention utilizes a raster scanned video signal obtained from a closed circuit television camera which sees the same image as would be seen by a manual operator of the system. Accordingly, adjustment and correlation of results between the manual and automatic systems methods is readily accomplished. This image is provided by a split-field microscope.
Alignment in both the manual and automatic modes is obtained when fiducial or reference marks on the silicon wafer are brought into alignment with corresponding indicia on a reticle in the microscope. The reticle is a reference point which is mechanically fixed within the system and which serves as the reference point for movement of the X-Y stage. It is a feature of the present invention that the final alignment of the wafer under automatic control is similarly with reference to the reticle and not to some position electronically established within the raster scan image. As is understood by those skilled in the art, the image of the reticle itself may shift within the video frame as the microscope is focused or due to some mechanical or electronic change in the closed circuit television apparatus.